A solar cell or photovoltaic cell is a semiconductor device that converts solar/optical energy of light into electrical power by the photovoltaic effect. Generally, a solar cell is configured as a large-area P-N junction made of silicon that has a layer of N-type (negative type) silicon and a layer of P-type (positive type) silicon direct contacting with the layer of N-type silicon. When a photon hits the solar cell, the photon can pass straight through the silicon if it has lower photon energy, or reflect off the surface, or be absorbed by the silicon if it has photon energy higher than the silicon band gap value. The latter generates an electron-hole pair and sometimes heat, depending on the band structure of the solar cell. Due to the interfacial electrical field of the P-N junction, the generated hole moves towards an anode on the P-type silicon layer, while the generated electron moves towards a cathode on the N-type silicon layer in the silicon solar cell, thereby producing electricity.
Materials used for solar cells include silicon, group III-V semiconductors (e.g., GaAs), groups II-VI semiconductors (e.g., CdS/CdTe), organic/polymer materials, and others. Of them, silicon solar cells including monocrystalline silicon wafer based solar cells, polycrystalline silicon (poly-Si) thin-film based solar cells and amorphous silicon (a-Si) thin-film based solar cells are most developed. Group III-V semiconductor based solar cells are formed on bulk Ge substrates and have high efficiency, but they are prohibitively expensive for all but applications in satellites and integrated optics, because the Ge substrate constitutes a large portion of this cost. Additionally, group III-V and II-VI semiconductor based solar cells can not be easily integrated with Si-based CMOS and glass panel TFT-LCD and LTPS process. Furthermore, there are issues of heavy metal pollutions in fabricating group III-V and II-VI semiconductor based solar cells. Although a-Si thin-film solar cells have low cost, they have low efficiency and instability. Therefore, silicon wafer based solar cells are dominant in the solar cell market.
Solar cells operate as energy conversion devices, and are therefore subject to the Camot Limit of conversion efficiency, which is about 85%. So far, the highest conversion efficiency of which market available solar cells have achieved is about 33%. Therefore, there are rooms for the improvement of the efficiency of the solar cells.
Theoretically, photons with an energy below the band gap of the absorber material cannot generate a hole-electron pair, and so their energy is not converted to useful output and only generates heat if absorbed. For photons with an energy above the band gap energy, only a fraction of the energy above the band gap can be converted to useful output. When a photon of greater energy is absorbed, the excess energy above the band gap is converted to kinetic energy of the carrier combination. The excess kinetic energy is converted to heat through phonon interactions as the kinetic energy of the carriers slows to equilibrium velocity. The solar frequency spectrum approximates a black body spectrum at about 6000 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat via lattice vibrations (phonons) rather than into usable electrical energy. For single junction (single band gap) solar cells, the highest conversion efficiency in theory is about 28%. However, the average conversion efficiency for monocrystalline silicon and poly-Si solar cells in the market is only about 15%, because of the intrinsic limitations of materials that are unable to absorb all incident photons with energy greater than the band gap, and of the free-carrier absorption of the materials that limit the 100% conversion of the photon absorption into electron-hole pairs.
For multi-junction (or multi-band gap) solar cells, which is a stack (tandem) of individual single-junction solar cells in descending order of band gap, the top cell captures the high-energy photons and passes the rest of the photons on to be absorbed by lower band gap cells. The use of multi-band gaps (or multi-junctions) can reduce the intra-band energy relation so as to reduce the probability of generating phonons, thereby reducing heat generation and improving the photovoltaic conversion efficiency, comparing to the single junction (single band gap) solar cells. However, the tandem solar cells have issues of junction loss and lattice mismatch.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.